The US Federal Bureau of Investigation (FBI) is now treating the deadly shooting in San Bernardino, California, last Wednesday as an act of terror.
This was as militant group Islamic State in Iraq and Syria (ISIS) yesterday said the couple who carried out the attack were its followers.
After two days of debate and speculation over whether the killing of 14 people in the city was terrorism or simply the act of a disgruntled worker, the authorities say the evidence is now mainly pointing at the former.
"There are a number of pieces of evidence which have essentially pushed us off the cliff to say we are considering this an act of terrorism," said Mr David Bowdich, the assistant director in charge of the FBI's Los Angeles office.
Though he did not offer details, a Facebook spokesman said that Tashfeen Malik - on the same day that she and her husband Syed Farook laid siege to a Christmas party - had posted on Facebook a pledge of allegiance to ISIS.
The posting was promptly removed by Facebook. Apart from the deleted posting, however, investigators have yet to turn up evidence that the duo had received training or instructions from ISIS. The couple died in a police shootout hours after the attack.
Said FBI director James Comey on Friday: "So far, we have no indication that these killers are part of an organised larger group or form part of a cell. There's no indication that they are part of a network."
Preliminary investigations suggested the duo were likely self-radicalised. Unlike the perpetrators of the terrorist attacks in Paris last month who were taking direct instructions from ISIS, the couple appeared to be - at least for now - merely inspired by the militants.
The New York Times, meanwhile, in its first front-page editorial in nearly a century yesterday, called for outlawing of the kinds of rifles used in the shooting. Referring to other mass shootings, it said these were "in their own ways, acts of terrorism". It blamed politicians for abetting would-be killers by creating gun markets for them.
As for Wednesday's attack, much remains up in the air, including why the couple chose to attack the Christmas party at Syed Farook's office and what the large stockpile of armaments found in his home was meant for. Investigators searching the home unearthed an unnerving cache of weapons, including thousands of rounds of ammunition and 12 pipe bombs.
Mr Comey said: "There is much about this that doesn't make sense... That is the reason we have hundreds of people running down leads all over the world on this and spending tremendous amounts of time as we sit here trying to understand the electronic record around these two killers."
Police are scouring through social media accounts and the phones and computers the duo left behind. The couple smashed a lot of their equipment in an apparent attempt to cover their digital footprints.
Mr Comey also urged Americans not to let the episode disrupt their lives. "What we hope you will do is not let fear become disabling but to, instead, try to channel it into an awareness of your surroundings, to get you to a place where you are living your life. But if you see something that doesn't make sense, you say something to somebody."
